PLATE XX. 



, Malm. 



1. External right aspect of a pair from Fitton Hall, Oldhum, Lancas 



2. of a pair from Bassenthwaite, Cumberland. 



3. a-d. of four pairs from Lough Fern, Co. Donegal. 



4. a, b. Posterior and dorsal view of a pair from Barnes Common, Surrey. 



5. ft, b. - of a pair from Lancaster. (" P. nitidiim var. splendens") 



6. , b. -- of a pair from Fair Head, Co. Antrim. 



7. , b. -- of a pair from Colchester (?), Essex. 



8. a, b. -- of a pair from Ziegenhals, Silesia. 



9. a-k. External aspect of a series of valves from the Holocene of tho 1 



Valley, Berkshire. 

 External right aspect of a pair from Barnes Common, Surrey. 



of a pair from Meigle Moss, Selkirk. 



of a pair from Hoe Moor, Hants. 



- of a pair from Borough Pond, Mortehoe, Devon. 

 External aspect of four valves from the Post-tertiary deposit at C< 



Essex. 



External right aspect of a pair from Lancaster. (" P. nilidui 

 splendens") 



of a pair from Fair Head, Co. Antrim. 



- of a pair from Colchester (?), Essex. 



of a pair from Ziegenhals, Silesia. 



Internal aspect of a series of pairs from Bishopthorpe, York. 



- of a pair from Bassenthwaite, Cumberland. 



of a pair from Borough Pond, Mortehoe, Devon. 



of a pair from Meigle Moss, Selkirk. 



External aspect of two valves from the Pleistocene at Clacton, Es 

 Diagrammatic enlargement of the Hinge. 



Internal aspect of a pair from Lancaster. (" P. nitidiim, var. splen 

 -- of a series of valves showing variation, from the Holocene 



Kennet Valley, Berks. 

 26*. rr, b. -- of two valves from the Post-tertiary deposit at Copford, Ess 



27. a, b. - of a pair from Menstrup, Sealand. 



28. a, b. -- of a pair from Fitton Hall, Oldham, Lancashire. 



29. a, b. -- of a pair from Hoe Moor, Hants. 



30. a-d. - of four valves from the Pleistocene at Clacton, Essex. 



[a is abnormal.] 

 - of a pair from Colchester (?), Essex. 



of a pair from Barnes Common, Surrey. 



of a pair from Lough Fern, Co. Donegal. 



of a pair from Fair Head, Co. Antrim. 



- of a pair from Ziegenhals, Silesia. 



*** The scale in millimetres under each figure, or series of figures, 

 shows the amount of magnification. 



